Rock wettability refers to a property that a fluid containing two immiscible phases could extend or be absorbed on a solid surface. The wettability of a rock surface such as hydrophilic or lipophilic property could reverse at a certain condition, and this reversal property is known as wettability reversal property. The change on rock wettability may directly have an effect on rock capillary pressure, oil-water relative permeability ratio, etc.
During drilling, wettability of the borehole wall rock has critical impacts on the stability of borehole wall. When rock surface is hydrophilic, free water in the drilling fluid would easily cause capillary phenomenon and permeable hydration so as to result in borehole instability. In addition, for low/ultra-low permeability reservoirs, it would be damaged very easily by invading of external fluid such damages as water blocking and water sensitivity, etc. In view of above problems, it is necessary to treat the rock surface by some methods to turn it into non-hydrophilic and non-lipophilic so as to prevent water and oil as well as capillary phenomenon, and then to maintain the stability of borehole wall and protect reservoirs.
Recently, there are mainly three types of wettability reversal surfactants for rock overseas, which are: (1) cationic surfactant, the most commonly used is cetyl trimethyl ammonium bromide (CTAB); (2) anionic surfactant, the most commonly used is polyoxyethylene (propylene) alkyl alcohol ether sulfate salt; (3) nonionic surfactant, the most commonly used is polyxyethylated alkylphenol. These surfactants do have wettability reversal function in some degree, but not sufficient to provide amphiphobicity to rock.